Phase-change material (PCM) memory, now commercially available, has been in use for non-volatile storage of digital data. Details and principles of operation of PCM memory are available elsewhere. While PCM memory has advantages and disadvantages relative to other forms of volatile as well as non-volatile memory, PCM writes are performed slowly relative to PCM reads, and PCM writes may be slow compared to writes of some other types of memory. Moreover, PCM writes require significant power because of the need to sufficiently heat a cell to change states. The power potentially needed for writing may also limit the write performance because as more writes are performed concurrently, more power is required; concurrency is limited by maximum power requirements. In addition, read performance can be impacted by write performance, because the longer that writes take to complete, the longer that pending reads must wait before being executed.
Various approaches have been taken to improve PCM memory. Because PCM memory has a limited lifetime write endurance, PCM memories have been designed such that when a write is to be performed, only the cells that would be changed by the write are physically modified. Additionally, caching techniques have been proposed to reduce the number of times that a cell is modified (set or reset). Nonetheless, improvements continue to be sought.
Techniques to improve PCM memory efficiency are discussed below.